Processes for the tanning of hides using zinc compounds



3,000,639 PROQESSES FOR THE TANNING F HIDES USING ZINC COMPOS Harlan A.Depew, Kirkwood, Mo, assignor to American Zinc, Lead & Smelting Company,St. Louis, Mo., a

corporation of Maine N0 Drawing. Filed Oct. 23, 1957, Ser. No. 691,799

9 Claims. (Cl. 8-94.25)

This invention relates to processes for the conversion of hides toleather by the use of zinc compounds that precipitate zinc oxide,hydroxide, or basic compounds in the hide and which combine with it andto the resulting products. This application is a continuation-in-part ofmy application Serial No. 429,672. filed May 13, 1954 now abandoned.

The prior art recognizes that zinc is desirable in leather manufacturingand patents have been issued for the use of zinc compositions as minoradditives in various stages of leather manufacture from dehairing to thefinished leather. in the prior art the hide invariably has been so acidfrom pickling during tanning that no significant amount of zinc can havebeen left in the leather.

In a known process, zinc compounds are deposited in the hide but theyare not tanning agents because the hide is fully tanned with chromiumbefore the zinc is added. The reactive groups of the hide are saturatedwith the chromium compounds. The tanning acid is neutralized with sodiumzincate, sodium aluminate, or an ammoniacal copper solution. Zinccompounds, aluminum cornpounds, or copper compounds are deposited in theleather. However, since the leather has been fully tanned prior to theaddition of the alkali zinc compound, these compounds are not added astanning agents and the reason for their use is not clear. Theprobability is that the zinc hydroxide or basic zinc compound helpedfill the leather, an expensive compound for this purpose.

Hides have also been impregnated in the prior art with the salts of therare earth metals and with zinc sulfate. The hide is then reacted withdisodium phosphate whereby the phosphates of the rare earth metals andthat of zinc are precipitated within the hide in the form of compoundsinert to the leather. In the present invention zinc is deposited in areactive hydrated form in the hide.

White leathers are known in the industry, and one common method toobtain white leathers is by alum tanning in which basic aluminumcompounds are deposited from an acidic solution. These products havevery poor dimensional stability at elevated temperatures and thealuminum compound leaches freely. Zirconium has also been used for thispurpose but is expensive and is not widely used.

White leathers presently known are so unsatisfactory that the mostcommon commercial method is to tan the hides with chromium; then surfacebleach the tanned hides and then white coat the leather which coating isfrequently scuffed off in use.

In contrast leather tanned with basic zinc compounds by the method ofthis invention is white all the way through. When the leather is scuffedoff the exposed leather is also white. When this white leather is put inboiling water it loses its white color presumably through reaction ofthe zinc and the protein; the zinc is not leached. This is not apractical consideration, however, since this white leather would not beused in boiling water. In normal storage there is no lossof whiteness.

I have found that the disadvantages of the prior art are overcome andwhite leathers are obtained by combining basic zinc compounds with thehide to tan it, giving a white leather with acceptable shrinktemperatures.

The present processes for the first time produce a White mates PatentedSept. 19, 1961 leather product by precipitating basic zinc compounds inthe hide where some of it combines with the hide and gives the desiredproperties of tanned leather and part of it is present as a pigmentwhitening the leather throughout. These white leathers are not onlyuseful in themselves but they serve as a desirable basis for tints andcolors.

Zinc hydroxide react readily with zinc chloride and zinc sulfate thatare added to form complexes known as basic zinc salts, XZn(OH) -ZnCl andYZn(OH) -ZnSO and ZZn(OH) -Zn(NO for example. These basic zinc compoundsinclude zinc hydroxide. Zinc hydroxide dehydrates to ZnO at relativelylow temperatures especially under alkaline conditions and is alsoincluded herein. The required chloride and sulfate is obtained not onlyfrom the added zinc chloride and zinc sulfate but also from sodiumsulfates and chlorides present in the hides.

It is, accordingly, an object of the present invention to provide novelprocesses for tanning hides to leather using basic zinc compounds forthis purpose.

Another object is to provide such processes in which zinc oxide, zinchydroxide and basic zinc compounds are deposited in the hides to tanthem.

Another object is to provide such processes in which the zinc compoundsdeposited in the hides combine With the hides in tanning them.

Another object is to provide such processes for producing white leather.

Another object is to provide novel white leather products tanned by zinccompounds.

Another object is to provide novel white leather products tanned bybasic zinc compounds partly combined with the fibers of the hides.

Another object is to provide novel white leather products produced bytanning hides with zinc hydroxide including zinc oxide, and basic zinccompounds deposited in the hides.

Other and further objects of the present invention will appear from thefollowing illustrative descriptions thereof.

A hide treated with basic zinc compounds according to the processes ofthis invention may act as if it had been bated, and accordingly, forsome grades of leather hating can be omitted and the hide limed withconventional dehairing processes can be treated directly With zincsolutions to precipitate zinc hydroxide andb'asic zinc salts therein. ifthe limed hide is treated with a zinc sulfate solution to precipitatezinc hydroxide, calcium sulfate also will be precipitated in the hide asa desirable filling material. Similarly, barium sulfate is precipitatedwhen barium hydrate is the base.

When zinc sulfate or zinc chloride is used with NaOH in the processes ofthis invention, Na SO or NaCl is formed. These salts, with added saltsas required, control the swelling of the hide.

The scraped and dehaired and limed hide either bated or unbated may godirectly to the tanning procedure of this invention if the hide has thenecessary a'cid neutralizing capacity.

The processes of the present invention are capable of various proceduralmodifications and illustrative embodiments thereof are hereinafterdescribed to illustrate the invention and the resulting products varysomewhat with the processes employed.

In general the pH of the liquid exuding from a hide before adding thesoluble basic zinc compounds including zinc chloride and zinc sulfateliesbetween 5 and It to give the hide a suitable acid neutralizingcapacity. The minimum pH at which, basic zinc compounds precipitatevaries with the zinc chloride concentration and other salts present.Ordinarily pl-Is of the liquor surrounding the hide higher than 11 areavoided as injurious o to the hide but there is no upper limit. Actuallya high pH is not essential. The COOH and other reactive groups of thehide tend to react with the soluble bases added -by the processes ofthis invention and give an acceptable acid neutralizing capacity withouta high pH.

The bases rather than basic salts that are used in this invention arewater-soluble compounds, thenegative ions being (GI-l). Among thesebases in addition to lime already mentioned are NaOH, KOH, LiOI-I, andBa(OH) Ammonium hydroxide is an especially useful base because aftertanning any excess can be evaporated. Watersoluble amine base such asdimethyl amine hydroxide and tetramethyl ammonium hydroxide are examplesof other bases, of which there are a number than can be used.

In carrying out this invention, the amount of zinc to be added isdetermined and the hide is then treated when necessary with a watersolution of a base in sufficient concentration to provide acidneutralizing capacity so that when a solution of the zinc saltis added,the zinc salt will diffuse into the hide and will precipitated therein.

The zinc salt and base may be added alternately to the hide when a highzinc content is desired. The order of adding is not essential, but thebase is usually added first.

In another embodiment of the processes of this invention a hide, if notbasic, is given an acid neutralizing capacity using dilute ammonia waterof, for example, 1/22% concentration. The hide is then soaked in anammoniacal solution of a basic or normal zinc salt such as the chlorideor sulfate, preferably with agitation and some pounding action as in atumbling mill. The addition of salt (NaCl) to the solution oftenimproves the zinc penetration in the hide. After the zinc compound hasentered the hide, the hide is removed from the solution and much of thefree ammonia is evaporated. It is generally accepted that the zincexists in the ammoniacal solution as Wernerian compounds such as (NHZnCl and As the ammonia evaporates, the zinc in the hide becomesinsoluble and precipitates, presumably largely as a basic zinc chloridebut in'part probably as zinc oxide, zinc hydroxide, and zincdiaminochloride (ZnCl -2NH Ammonium salts and other salts may also formin the hide. The treated hide is then washed with water or with diluteammonia or soda ash. The-hide is then oiled and may be further processedin known manner with other tanning materials, such as syntans,formaldehyde, or chromium compounds, but this last step is unnecessary.

X-ray studies have indicated that basic zinc chloride such as isprecipitated in the hides by the processes of this inventioncrystallizes in flat hexagonal plates with the zinc atoms in one planeand with the Cl atoms and OH groups oriented in an adjoining plane. Ineach location the presence of an OH radical or a Cl atom is a randomarrangement. The percentages of Zn, OH, and C1 in the compound may varywidely. A well recognized ratio is expressed by the empirical formula--4Zn(OH) -ZnCl The ratio of four Zn(OH) to one ZnCl is the basicity. Asthe basicity increases, the maximum amount of zinc in solution decreasesand the stability of the solution becomes poorer. For this reason whenhigh concentrations of zinc chloride are desired in the hide it ispreferable to use solutions of lower basicity as represented by2Zn(OI-I) -ZnCl Zn(OH) -ZnCl or even lower ratios.

The one extreme of basicity is essentially pure zinc hydroxide dissolvedin ammonia, and the other extreme is zinc chloride dissolved in ammoniaand NH Cl.

When zinc chloride is dissolved in aqua ammonia and the solution isevaporated, a large number of compounds are precipitated ranging fromzinc hydroxide through a range of basic zinc chlorides to ammoniumchloride and the mixture will analyze approximately ZnCl -ZNH .zincdiaminochloride. In the case of Zn(OH) dissolved i in aqua ammonia andprecipitated, the deposit is largely Zn(OH) Although the deposit onevaporation of ammoniacal basic zinc chloride solution analyzeapproximately xZn(OH) -ZnCl 2NH X-ray studies and stability testsindicate that ammonia with water is absorbed on the basic zinc chloride,rather than as an essential part of the crystal.

In any case, there is available approximately two NH for each ZnCl inboth the zinc diaminochloride and in the basic zinc chloride.

When hides containing zine diarninochloride in the residue fromammoniacal basic zinc chloride solution are washed, the compoundshydrolyze and the end product tends toward Zn(OH) The basic zinccompound before hydrolysis in and around the fiber in part reacts withthe fibers of the hide and converts the hide to leather andthe remainderof the basic zinc compounds deposits around the fibers whitening,separating and lubricating them.

When a hide treated with an ammoniacal zinc solution is partially driedin the air, the free ammonia concentration is greater on the interior ofthe hide than on the surface. The hide may then be treated with anacidic zinc salt solution to deposit less basic zinc compounds in thehide. The hide is then leached and NH Cl is removed leaving basic zinccompounds in the hide.

In general, increasing the amount of precipitated zinc in the hideincreases the leather qualities of the hide and approximately %-1% ofprecipitated zinc are needed to make the treated hide show pronouncedleathery qualities. However much larger quantities of precipitated zincmaybe used.

Zinc oxide and zinc hydroxide are very insoluble and Lhgir basiccompounds produce a permanently tanned The processes described aboverefer especially to basic zinc compounds containing chloride, but otherzinc compounds such as the sulfates, nitrates, etc. may be used.

White zinc tanned leather produced by these processes may be dyed todesired colors or it may be reacted with chemicals to develop uniformcolors through the leather.

Leather tanned with zinc according to these processes has unusually goodtensile strength and tear resistance attributable to the avoidance ofthe highly acid solutions that are used in conventional tanning.

These processes and the resulting products are illustrated by thefollowing examples in which a moist basic zinc chloride was dissolved in2628% aqua ammonia to form the solution:

Percent Zn -s 9.0

Cl 3.1 N-H approx -s 21 Evap. residue approx 17 Example I The solutionabove was diluted with three parts of water and a oalfbelly prepared asinExample I was immersed in it for 18 days. During this period thetreatment was carried on out of doors and the liquid froze and thawedand some ammonia was lost. The treated hide was then dried for tenhours-and leached and oiled. The product is designated hereinafter as B.

Example Ill The solution above was diluted with 7 parts of water to makea solution having 1.1% Zn. The reduced ammonia content makes thesolution less stable and with further dilution zinc hydroxide wouldprecipitate. However, at this concentration, there is no precipitate. Athird calfskin belly was immersed in this solution simultaneously withthe calf skin in Example 11. It was removed in the same length of timeand dried, washed, oiled, and dried as in Example II. This product isdesignated hereinafter as C.

Within less than 24 hours after the hides were inimersed in the treatingsolution, an increase in stillness of the hide was noticeable in A andB.

X-ray examination of C showed no ZnO, Zn(O l-l) ZnCO NH Cl, zincdiaminochloride, nor basic zinc chloride, although small amounts may bepresent, but showed a diffuse unknown pattern that could only be azinc-protein compound. A and B contained, in addition, zinc hydroxidelines.

The three leathers were white and flexible and withstood boiling waterwith less shrinkage and hardening than a piece of commercial alum tannedleather.

Zinc analyses of these leathers were:

, Percent Zn A 1.4

27 parts of ZNSO -H O were dissolved in 200 parts of water. 100 parts of26-28% NH (aq.) were stirred in to obtain a colorless solution that wasdiluted to 500 parts with water. The calf-belly hide was ball milled inthis solution for 6 hours and was allowed to stand in the millingsolution for another 10 hours. The hide was equeezed by hand to removeexcess liquid and was hung to air dry for 5-5 /2 hours.

The hide was milled for 6-6 /2 hours in a solution consisting of 13.7parts ZnSO -H O and 237 parts water. It was next rinsed and washed wellwith water and finally milled for 3 hours in 200 parts of 1:1 sulfonatedoil water emulsion. The oiled hide was rinsed with water and allowed toair dry until the required dryness was reached to obtain a moderatelystifi white leather.

Example V Bated calf-belly hides were soaked in 2% NH water to preservethem from bacterial decomposition and to allow uniform penetration ofthe NH A 6" x 12" piece of this hide was soaked with occasional stirringfor 24 hours in a solution consisting of 41.4 parts ZnSO -6H O in 460parts of water. The hide was removed, allowed to drain to remove excesssolution, and was hung to dry in air for 15-20 hours. While still moist,the hide was further dried for 3-4 hours in a circulating air oven at40-41. The hide was next soaked in water overnight then was milled for 8hours in a ball mill containing equal parts of water and Nopco alum oil(anionic surfactant). After further soaking overnight in the oilemulsion, the hide was removed, rinsed lightly with water, and air driedat 25-30 C. The final dry product was a well tanned, white leatherhaving excellent pliability and strength.

Example VI A 5" x 8" piece of NI'Ig'SOELkCd, bated calf-belly hide wasplaced in a ball mill with a solutionconsisting of 25 parts ZnCl and 475parts water and was milled with porcelain balls for 7-8 hours. The hideafter standing in the milling solution was removed and squeezed to re-'move excess ZnCl The hide was returned to the mill and was milled for77% hours in a slurry comprising 50 parts slaked lime and 450 partswater. The hide was next washed with water, and was allowed to soakovernight in a solution of 25 parts ZnCl and 475 parts water. The hidewas removed, rinsed in running water for 15 minutes, and was milled for4 hours in an emulsion consisting of 250 parts Water and 250 parts ofNopco alum oil. After rinsing with water, the hide was air dried toobtain a white leather having good pliability and no visible rawhide oncutting. Analysis of the air-dried leather shows 1.4% Zn.

Example VII A 5" x 12" piece of limed but unbated calf-shoulder hide wasstored overnight in a solution consisting of 165 parts ZnSO -H O, 75parts of salt, and 1260 parts water. The hide then was milled in the ZnSO solution for 2 hours and was allowed to stand in the milling solutionfor another 35-40 hours. It was next rinsed in water and squeezed toremove excess ZnSO After milling for 4 hours in an emulsion consistingof 100 parts Water and 100 parts Nopco alum oil, the hide was rinsedlightly with water and was hung to air-dry. The final dry leather had agood opaque white color and analyzed 3.8% Zn.

Example VIII A piece of hide was cut from the shoulder area of bated cowhide, approximately 4" x 6" with a wet drained weight of 73 g. The hidewas washed in running tap water and then placed in a S-qt. ball millabout half full of V2? porcelain balls in a solution of 1381 cc. ofwater, 107 cc. of 28% NH (2% water soln.), 112.5 g. NaCl (7.5% soln.)and 30 g. of NH Cl as a butter to maintain a pH of 10.5. It was ballmilled for 5% hours. The pH of the hide surface at this time was 10.5.

The hide was now placed in a l-qt. ball mill about half full of /2balls. Solutions of 5.1 g. (7% of wet hide weight) of NaCl, 7.3 g. ZnSO-H O (10% of wet hide Wt.), enough 5 N. H 50 to bring the pH to 2, andthen 2.2 g. (3% of wet hide weight) of sodilun citrate were added andenough water in the solution to cover the hide in the ball mill. Thefinal pH of solution was 3. The hide was ball milled 1% hours and the pHincreased to 5.5. The hide was left standing in the mill for 36 hours.It was ball milled an additional 1% hours and the pH reach 6.2.

The milling solution was poured into a beaker to which oils wereadded-8.8 g. of Tanoyl 1881 (12% of wet hide wt.) and 2 g. of NopcoleneK.Q.W. The solution was heated to C. and stirred. When the oil was welldispersed, the solution was poured back into the mill with the leatherand the leather was ball milled for 4 /2 hours. Then the leather wasremoved and lightly washed in running water, and hung to air dry at roomtemperature.

The wet tanned hide dried into a slightly stiff very good white leatherwith a shrink temperature of 79 C. compared with about 52 C. for the rawhide.

The oils were furnished by Nopco Chemical Co. and are described by theproducers as follows: Tan Oil 1881 is anionic with a pH of 6 /2-7. It isa slightly bleached sulfated sperm and COFA oil intended for whitechrome. Nopcolene K.Q.W. is non-ionic cationic. It is a mixture of blownvegetable oil, sulfonated neat and sperm oil, wax, and tallow. It is afulling type of oil. It makes the leather feel thicker and spongy.

Example IX A 14" x 14" square piece of bated rawhide cut from the flankportion of a cow hide was washed and rinsed thoroughly in running tapwater. It was wet drained and the weight was 1105 g. The hide was placedin a S-qt. ball mill about half full of pebbles. A solution of 1 literof water, 75 g. common salt, and 5 cc. of cone. NH (28%), was added. Itwas milled 5 /2 hours, then stopped, and .left standing :in themill-overnight. .It was milled an additional hour the following morning.The solution pH at this time was 95-10. This solution was discarded. Thehide Was placed back in the ball mill in a fresh solution of 1. I.water, 110.5 g. (10% of Wet hide wt.) ZnS -H O, 77.4 g. (7% wet hidewt.) NaCl. The pH was adjusted to 3 with addition of N. H 80 and 4-5 g.of sodium citrate were added as a buffer. The hide was ball milled 3hours, stopped and 111 cc. of 40% HCHO (4% HCHO of wet hide wt.'), and5-6 g.'ZnO to buffer the solution at pH 5 were added. It was ball milled3 hours, left in the mill overnight and ball milled an additional hourthe following morning.

The milling solution was poured into a beaker and 166 g. of Nopco Tanoyl1881 of wet hide wt.) were added with 40 g. of Nopcolene K.Q.W. Thesolution was warmed to 80 C. and stirred until the oil was welldispersed. The oiling solution was poured-back into the ball mill on thehide and the ball milling continued for 5 hours.

The hide was removed, rinsed in running tap water and hung to air dry atroom temperature overnight. The following morning the hide was placed ina circulating oven and finished drying at 60 C.

The dried hide was slightly stiff. Upon working, the leather softened.The shrink temperature was 88 'C. A sample of the leather analyzed 2.51%Zn.

area of a cow hide that had been limed, dehaired, and

bated in the regular factory procedure used in preparing hides forchrome tanning. The exuded'liquor had a pH of approximately 7 but it hadan appreciable amount of acid neutralizing capacity that was capable ofreacting with soluble zinc salts. and placed in a S-qt. ball mill abouthalf full of pebbles in a solution of 1 l. or" water, 92 g. 10% of .wethide wt.), ZnSO -H O, 64 g. (5% of Wet hide wti), salt. The pH of thesolution was adjusted to 3 with 5 N. H SO and buffered with 5.6 g. ofsodium citrate. The hide was ball milled 6 hours, when there was added92 cc. of 40% HCHO (10% of wet hide wt.) and 5-6 g. ZnO to increase thepH to 5. The hide was ball milled 1 hour then left in the millovernight. It was ballmilled an additional hour the following morning,then removed and discarded the solution. At this point the hide had ashrink temperature of 90 C.

The hide was returned to the ball mill in a solution of 1 1. water and30 g. of Neil- S0 and ball milled for 2% hours to removeexcess HCHO.[The solution was discarded at the end of the ball milling period.

A fresh solution of 750 cc. H2O,92 g. ZIISOgH O,

64 g. salt and 183 g. of Tanoyl 1881 of wet hide wt.) was prepared. Thesolution was heated to 70 C., stirred until the oil was dispersed well,and then poured into the ball mill on the hide. It was ball milled'2hours, removed and rinsed in tap water andhungto air dry at roomtemperature. The dried leather became softer and whiter upon working. Ithad a shrink temperature of 84 C. and analyzed 3.29%.Zn.

The NaHSO treatment reduced the added tan given by the formaldehyde.

Example XI A 14" x 14" piece of hide out from the shoulder area of abated hide, wet wt. 780 g., was rinsed thoroughly in tap water andplaced in a S-qt. ball mill, half full of pebbles, in a solution of 1 l.of water, 5 cc. of 28% NH and 55 g. NaCl (7% of wet hide wt.). The pH ofthe solution was 10-10.5. The hide was ball milled 4% hours and leftstanding in the ball mill overnight.

The hide was removed from the ball mill the following morning andthesolution was discarded. A fresh solution of 1 l. of water, 78 g. (10%wet hide wt.)

ZnSO 'H O, and 55 g. (7% hide wt.) .NaCl was pre- The hide was rinsedthoroughly,

pared in the ball mill. The hide was placed in the ball mill and the pHwas adjusted to 3-4 with 5 N. H SO and sodium citrate buffer. After ballmilling for 2% hours, the solution pH was 4-5, and 5-6 g. of ZnO wereadded to give a pH of 5.5-6. The hide was ball milled 3 hours and leftstanding overnight.

The following morning 177 g..of Tanoyl 1881 (15% of hide wt.) were addedto the milling solution which was removed from the ball mill and warmedto 60 C. with stirring until the oil was dispersed. The solution waspoured back into the mill and the ball milling continued for 3 hours.The hide was removed, washed in lukewarm water .to remove excess oil andhung to air dry at room temperature.

The leather dried to an off-white which improved upon working. It had ashrink temperature while still wet of 73 C., and upon drying the shrinktemperature was 68 C. The Zn analysis was 7.03% Zn. After 24-hourleaching in distilled water a sample analyzed 5.3% Zn.

Example XII A 4 x 6" (77 g. wet wt.) piece of bated rawhide cut from theshoulder area of a large cowhide, was rinsed in running tap water andthen placed in a 'l-qt. ball mill with about 2 doz. pebbles. Enoughwater was added to cover the hide (approx. 100 cc.), together with 7.7g. (10% wet hide wt.) ZnSO -H O, and 5.4 g. NaCl (7% hide wt.) 5 N. H 80was added to a pH of 2, and 2.3 g. (3% of hide wt.) of sodium citratethen added to buffer at a pH of 3. The hide was'ball milled 4 /2 hou-rs,allowed to stand overnight in the milling solution and then ball milled5 additional hours. At this point the pH of the solution was raised to 5with the addition of NaOH. The ball milling] was then continued for oneadditional hour.

The tanning solution was poured into a beaker and 9.2 g. Tanoyl 1881(12% of wet hide wt.) and 10 g. of Nopcolene K.Q.W. were added. Themixture was warmed to 60 C. with stirring until the oil was welldispersed, and then poured back intothe ball mill on the leather. Theleather was milled for 2 hours, left standing overnight and then milledan additional 1 /2 hours. The hide was then removed, rinsed, and-hung toair dry at room temperature.

The leather dried soft, white, and very pliable. It analyzed 3.55% Zn.

In general, zinc treated leathers tanned at the lower pI-Is show aslightly lower shrink temperature and a brighter white with superiorflexibility and less fullness. I have found that I can immerse thetanned hide in a solution mildly alkaline with NaOH or the other basesdescribed in paragraph 2 of page 5 for example without loss in desirableproperties. I have used solutions having pHs of 10.5 to 8.0 for this endpointing but I prefer the latter although the exact pH is not critical.The time of treatment can vary from a short period to several days. Someoil in the end pointing solution is desirable to prevent leaching of oilfrom the tanned leather.

Alkaline endpointing increases the shrink temperature, especially forlow shrink temperature leathers.

Although the zinc in basic zinc compounds is extremely insoluble in purewater, in the presence of salts such as NaCl and Na SO complexes formthat may cause some loss of zinc. Where this loss occurs the describedalkaline end-pointing reduces this zinc loss.

Unlike ordinary leather, the zinc tanned leather of this invention canbe fastened with metal fasteners and wet with water and the leatherisresistant to discoloration.

Further, the leather of the present invention upon repeated experimentshows no mildew attack even in warm Weather when commercial leathersused as the control are heavily covered by a slimy growth. By mildew isincluded all fungi and lower forms of life that form these disagreeablecoatings. This high mildew resistance is due to the basic zinc compoundswith a range of low basicity to high basicity.

The leather of this invention has been examined by many experts inleather technology and they agree that the zinc tanned leather of thisinvention has unusual and superior strength as shown by hand tearing.The fibers of the leather can be separated as strong, individual fibers.The Zinc compounds have not only tanned the leather by combining in partwith the protein thereof but they have coated and whitened the fibersand kept the fibers separate and lubricated. I attribute the strength ofthe leather in part to the fact that the hide has not had to undergo thesevere acid attack characteristic of tanning where pickling is used andof chrome tanning in particular.

It should now be apparent that the present invention provides novelprocesses for the tanning of hides using zinc compounds with resultingnovel products which in every way satisfy the objects described above.

Changes in or modifications to the above-described illustrativeembodiments of this invention may now be suggested -to those skilled inthe art without departing from the present inventive concept andreference should be had to the appended claims to determine the scopethereof.

What is claimed is:

1. In a process for tanning hides the steps of soaking the hidesprepared for tanning in an aqueous solution of a base selected from thegroup consisting of Ca(OI-I) NaOH, KOH, LiOH, Ba(OH) NH OH,dimethylamine hydroxide and tetramethyl amonium hydroxide having analkalinity not above an approximate pH of 11 and then soaking the hidesin an aqueous solution of a zinc salt selected from the group consistingof zinc chloride, Zinc sulphate, zinc nitrate, a solution of zincchloride in ammonia, a solution of basic zinc chloride in ammonia, asolution of Zinc sulphate in ammonia, and a solution of basic zincsulphate in ammonia and precipitating basic zinc compound in the hidesby reacting in the hides the base with the zinc salt forming zinchydroxide which reacts with the zinc salt to form basic zinc compoundswhereby the hides are tanned.

2. A process as described in claim 1 the additional step of soaking thetanned hide in an aqueous solution of a base selected from the groupconsisting of Ca(OH) NaOH, KOH, LiOH, Ba(OH) NH OH, dimethylaminehydroxide, and tetramethyl ammonium hydroxide.

3. In a process for tanning hides the steps of soaking basic hideshaving an alkalinity not above an approximate pH of 11 in an aqueoussolution of a zinc chloride and precipitating zinc compounds in thehides by reacting the base in the hides with the zinc chloride formingzinc hydroxide which reacts with the zinc chloride to form basic zincchlorides whereby the hides are tanned.

4. A process as described in claim 3 the additional step of soaking thetanned hides in an aqueous solution of a base selected from the groupconsisting of (Ba(OH) NaOH, KOH, LiOH, Ba(OH) NH OH, dimethylaminehydroxide and tetramethyl ammonium hydroxide.

5. In a process for tanning hides, the steps of soaking the hidesprepared for tanning in an aqueous solution of a normal zinc saltdissolved in aqua ammonia having an alkalinity not above an approximatepH of 11, and precipitating in the hides basic zinc compounds byevaporation of ammonia. F

6. In a process as described in claim 5, the additional step of soakingthe tanned hides in an aqueous solution of zinc chloride.

7. In a process for tanning hides the step of soaking basic hides havingan alkalinity not above an approximate pH of 11 in an aqueous solutionof zinc sulphate thus precipitating zinc compounds in the hides byreacting the base in the hides with the zinc salt forming zinc hydroxidewhich reacts with the zinc salt to form basic compounds whereby thehides are tanned.

8. In a process for tanning hides and the step of soaking basic hideshaving an alkalinity not above an approximate pH of 11 in an aqueoussolution of zinc nitrate thus precipitating the zinc compound in thehides by reacting the base in the hides with the zinc salt forming zinchydroxide which reacts with the zinc salt to form basic zinc compoundswhereby the hides are tanned.

9. In a process for tanning hides, the steps of soaking the hidesprepared for tanning in an aqueous solution of a basic zinc saltdissolved in aqua ammonia having an alkalinity not above an approximatepH of 11 and precipitating in the hides basic zinc compounds byevaporation of ammonia.

References Cited in the file of this patent UNITED STATES PATENTS281,411 Smallridge July 17, 1883 915,638 Trenckmann Mar. 16, 1909FOREIGN PATENTS 508,960 France Oct. 28, 1920 OTHER REFERENCES Cohn etal.: Separation of Protein Components of Human Plasma, J.A.C.S. 72 (Jan.1950) pp. 465-474.

Progress in Leather Science: 1920-1945 British Leather ManufacturersAssoc. London, 1948, pp. 527-529.

UNITED STATES PATENT OFFICE GERTIFICATE OF CORRECTION Patent No..3,000,689 September 19, 1961 Harlan A Depew It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 3, line 1, after no" insert sharp same column 3, lines 1 and 2,strike out "Actually a high pH is not essential."; line 74, for "ZnCl-ZNH read ZNCL -2NH column 4, line 89, for "chloride" read chlorinecolumn 5, line 35 for "ZNSO H O" read -ZnS.O -H O column 7 line 5, for"l. 1." read 1 16 Signed and sealed this 17th day of April 1962.

(SEAL) Attest:

ESTON G JOHNSON DAVID L. LADD Attesting Officer Commissioner of Patents

1. IN A PROCESS FOR TANNING HIDES THE STEPS OF SOAKING THE HIDESPREPARED FOR TANNING IN AN AQUEOUS SOLUTION OF A BASE SELECTED FROM THEGROUP CONSISTING OF CA(OH)2, NAOH, KOH, LIOH, BA(OH)2, NH4OH,DIMETHYLAMINE HYDROXIDE AND TETRAMETHYL AMONIUM HYDROXIDE HAVING ANALKALINITY NOT ABOVE AN APPROXIMATE PH OF 11 AND THEN SOAKING THE HIDESIN AN AQUEOUS SOLUTION OF A ZINC SALT SELECTED FROM THE GROUP CONSISTINGOF ZINC CHLORIDE, ZINC SULPHATE, ZINC NITRATE, A SOLUTION OF ZINCCHLORIDE IN AMMONIA, A SOLUTION OF BASIC ZINC CHLORIDE IN AMMONIA, ASOLUTION OF ZINC SULPHATE IN AMMONIA, AND A SOLUTION OF BASIC ZINCSULPHATE IN AMMONIA AND PRECIPITATING BASIC ZINC COMPOUND IN THE HIDESBY REACTING IN THE HIDES THE BASE WITH ZINC SALT FORMING ZINC HYDROXIDEWHICH REACTS WITH THE ZINC SALT TO FORM BASIC ZINC COMPOUNDS WHEREBY THEHIDES ARE TANNED.